In one type of robotically-assisted surgical procedure, a cutting implement (for example to cut bone) is mounted on an adjustable robot head which is itself held in position by a static gross-positioning device. The robot head has a manually-graspable handle which allows the surgeon to move the cutter. Typically, the cutter may be mounted at the end of a telescopic arm, and by applying force to the handle the surgeon may cause the arm to extend and/or to rotate about mutually-perpendicular pitch and yaw axes. Motors within the head respond to forces applied to the handle to ensure that the cutter moves smoothly to the position the surgeon requires. The head may include constraint mechanisms, implemented either in hardware or in software, which prevent the surgeon from moving the cutter into regions which have previously been defined as unsafe. Force feedback mechanisms may also be provided so that the surgeon receives tactile force feedback through the handle.
Of particular importance in surgical applications—although it may be of importance in other applications as well—is the precision with which the cutter can be positioned by the surgeon. Current systems are somewhat limited in this respect, because of relatively high friction in the mechanical components, along with a certain amount of “play” or backlash. A further requirement of course is safety, and concerns have been expressed as to the potentially serious injuries that could be caused to a patient in the event of a mechanical failure of a traditional robot head, or a failure in the control system or its software.